Various kinds of cooling systems for V-type, water-cooled, internal combustion engines have been developed in the past. Such a cooling system typically includes a water pump for circulating coolant, such as cooling water, through the engine. In general, the water pump is located in front of a lower center portion of a front end surface of the engine body. Cooled water flows from a radiator into an inlet port of the water pump, passing through a radiator outlet pipe and a suction pipe located at the front of one of right and left cylinder banks of the engine body. The water pump pressurizes the cooling water and forces the pressurized cooling water to flow around cylinders of the right and left cylinder banks through a connecting passage located in front of the right and left cylinder banks. The cooling water then passes up through cylinder heads of the right and left cylinder banks on its way out of the engine. The water, after leaving the engine body, returns into the radiator through an engine outlet passage and a radiator inlet pipe and has its temperature, which was increased by the heated engine, reduced by the radiator. The cooling system typically has a thermostat for providing constant temperature control. The thermostat is typically installed in front of the engine body, at a juncture between the radiator outlet pipe and a bypass connecting the radiator outlet pipe and the radiator inlet pipe. As is apparent from the above, it is usual in laying out the cooling system to locate both the thermostat and the suction pipe in what is termed a "dead space" left in one of the right and left cylinder banks between the front end of the engine body and the foremost cylinder. Such dead space is formed because one of the right and left cylinder banks is offset in the forward direction, with respect to the vehicle, relative to the other cylinder bank. Such an arrangement of a cooling system is known from, for instance, Japanese Utility Model Application No. 59 - 41150, entitled "Cooling System For V-Type Engine," filed on Mar. 22, 1984, and laid open as Japanese Unexamined Utility Model Publication No. 60 - 153818 on Oct. 14, 1985.
The cooling system as described in the above publication necessarily has a suction pipe, a bypass, a connecting passage and radiator inlet and outlet pipes, all of which must be great in length. This results in a bulky and messy cooling system and a complicated structure of the engine body around the pipes. Furthermore, since long pipes generate a great increase in friction drag on the cooling water, a water pump having a large size and water delivery volume must be installed.
Because the thermostat is located below the radiator inlet pipe, the cooling water, which is quite hot when leaving the engine body, is not expected to flow up the bypass by free convection. For this reason, since the water pump has only a low delivery capacity when the engine operates at a low speed and, accordingly, the circulation speed of the cooling water is low, it is difficult to keep the cooling water in the engine body at a constant temperature.
In addition to the above, because a front cover is attached to the engine so as to cover and protect a timing belt for coupling pulleys of valve driving camshafts and an engine crankshaft, heat, generated by the engine itself or due to friction between the timing belt and the pulleys, is retained in a space between the engine front cover and the front end of the engine. The heat lowers the service life of the timing belt. The complicated arrangement of various pipes makes it considerably troublesome to remove air in the cooling system.